A cryovolcano is an icy volcano found on icy bodies, especially moons, in the outer solar system. Cryovolcanoes have been observed directly on Neptune's moon Triton, during a Voyager II fly-by in 1989, and on Saturn's moon Enceladus, by the Cassini probe on 27 November 2005. Indirect evidence of cryovolcanism has been found on several other moons and bodies, including Europa, Titan, Ganymede, Miranda, and the trans-Neptunian object Quaoar.
Instead of erupting molten rock, as in a conventional volcano, cryovolcanoes erupt volatiles (low boiling point elements or compounds), like water, ammonia, carbon dioxide, nitrogen, or methane, accompanied by gas-driven solid fragments. This is called cryomagma. A cryovolcano produces plumes that may be a hundred or more degrees hotter than the frozen surface matter. Exposed to the cold and vacuum of space, the plumes quickly solidify, becoming airborne dust. As the gravity is weak on many ice moons, the plume may completely escape the moon's gravity well, go into an orbit, or crash back down on the surface in another area.
The source of energy of a cryovolcano usually comes from tidal friction, heat that builds up in the core of moons as they bend and distort in the gravity field of the massive gas giants they orbit. It is also suspected that some moons may have translucent layers of ice that permit light in to heat material beneath it, but have an insulating property that seals in heat and creates a greenhouse effect. This creates pressurized gases in the interior that will escape if there is a route to the surface, thus creating a cryovolcano.
Evidence of a cryovolcano was first uncovered on the south pole of Neptune's moon Triton, in the form of light streaks of nitrogen on the surface covering a dark, older crust. As the surface of some moons may be many hundreds of millions of years old, it can be easy to spot surface deposits that are relatively new, sometimes even just a few hours old. However, this does require direct examination by space probes doing fly-bys. These features are too small and distant to be seen by Earth-based telescopes or observatories.